1. Field of the Invention
The invention relates to a method of damping the rebound of print hammer magnets in typewriters or similar office machines, where the typewriters or similar office machines have a programmable control unit consisting of at least a microprocessor as well as a memory preferably divided into a ROM and a RAM and where the printing of a character is accomplished by charging the print hammer magnet with a specified first voltage curve U.sub.1 over a specified first time t.sub.1 and the damping of the rebound is accomplished by a braking pulse with a specified second voltage curve U.sub.2 over a specified second time t.sub.2 by means of the control unit
2. The Prior Art
Machines of the type dealt with in this application often have so-called adding print members, the characters of which are individually accelerated in the direction of the platen by means of a printing hammer magnet and thus make an impression in a known manner. Control of the printing hammer magnet in the known machine is done by means of a programmable control unit which customarily consists of at least one microprocessor and a memory, the memory being divided into a ROM containing the control programs and a RAM receiving the variable data. Depending on the character size the microprocessor controls a drive circuit which charges the coils of the printing hammer magnet with a specified voltage for a specified time. In this way a uniform print format regardless of character size is assured.
In connection with print hammer magnets a problem arises in that the armature of the print hammer magnet returns to its initial position with very high kinetic energy after the actual impression. Therefore steps must be taken to absorb this kinetic energy in such a way that both the noise generation and the damping period of the armature are minimized.
To solve this problem it is known, aside from the use of mechanical damping means, to achieve damping by charging the coil of the print hammer magnet with a braking pulse. For example, it is proposed in German Laid-open Application DE OS No. 26 45 498 to have the printing hammer magnet accelerate the characters towards the platen with variable speeds, depending on the character size. The variation in speed is achieved by a variation in the voltage with which the coils of the printing hammer magnet are charged. To damp the rebound in proportion to character size it is further proposed to brake the armature returning to its initial position by charging the coil of the printing hammer magnet with a percentage of the acceleration voltage. This method assures that a satisfactory rebound damping is achieved when the printing hammer magnet is charged with a braking pulse at the correct time. However, this correctly timed charging is highly problematical since, depending on the mechanical conditions, the entire repulsion procedure requires various amounts of time.
For the correctly timed pick-up of the braking pulse, German Patent DE PS 31 16 402 discloses a device which determines, by means of a sensor, for example in the form of a photoelectric barrier, the time when the returning armature of the print hammer magnet passes a specified point in its path. After a specified delay time the coil of the print hammer magnet is charged with the braking pulse. Although it is possible to provide the print hammer magnet with the braking pulse in a correctly timed manner by means of this device, this can only be achieved by the use of a relatively expensive sensor.